Thermoplastic (hot melt) inks for decorating vitreous, glass-ceramic, and ceramic ware are articles of commerce. Compositions therefor have been numerous with the majority thereof exhibiting wax-like characteristics with low melt viscosities, typically less than about 150 poises at their application temperatures, and tack-free surfaces when cooled to a significant extent below the softening points thereof. The inks or pastes used in decorating and/or printing have generally consisted of a vitrifiable, inorganic material or flux containing a suitable inorganic pigment or colorant and an organic, flowable vehicle or binder in which the pigment is dispersed. These pigment-containing flux materials have been utilized for many years for decorating glass and ceramic ware and are marketed by ceramic color manufacturers as complete vitrifiable or ceramic colors. These materials are dispersed in a vehicle or medium as desired by the user.
Inks suitable for decorating glass, glass-ceramic, or ceramic surfaces are typically filled with from about 60-90% by weight total of glass fluxes, inorganic pigments, and inorganic opacifiers. Such inks, after application, are fired at elevated temperatures (in excess of .about.500.degree. C.) to burn out the organic constituents and melt the glass flux. Accordingly, the organic constituents must be capable of being burned off at relatively low temperatures such that they are removable without adversely affecting the visual appearance and permanence of the decoration. Materials useful therefor have included waxes such as fatty alcohols, polyethylene glycol, polyethylene glycol esters, etc., fortified with a cohesive strength imparting resin or viscosity modifier such as ethyl cellulose, hydroxypropyl cellulose, butyl methacrylate, etc.
The application of thermoplastic inks to various substrates has generally taken the form of screening directly onto a substrate, although other practices, for example, offset screening from an elastomeric surface such as is disclosed in U.S. Pat. No. 3,688,695, have been described. The ink compositions reviewed above have been found satisfactory for use in screening processes but have proven less than fully adequate in other decorating techniques, particularly in those techniques utilizing elastomeric transfer (offset) operations such as offset screening or offset stamping from an intaglio plate. Such a process is described in U.S. Pat. No. 3,756,165. The operating parameters of these offset printing processes were necessarily governed by the properties of the conventional oil-based or solvent-based decorating inks. Such inks must be permitted to dry to a tacky, semi-solid state upon the elastomer surface following which they can be transferred as an integral film to a substrate. The glass fluxes and pigments exert a pronounced effect upon the viscosity of the organic vehicle or medium. Consequently, relatively high levels of organic solvents must be employed as carriers, which solvents should display reasonably high rates of volatilization so as not to unduly limit process speeds and/or saturate the surface of the elastomer. On the other hand, solvents demonstrating high rates of evaporation have given rise to viscosity stability problems of such severity that undesirably slow process speeds have been reluctantly accepted. This situation is disclosed in U.S. Pat. No. 3,756,165.
It is apparent that thermoplastic inks could alleviate this viscosity problem. Unfortunately, however, the formulations customarily employed in screening operations were less than satisfactory for transfer (offset) printing because of a lack of pressure sensitivity at temperatures much below their softening points. Stated in other words, the inks required heating to temperatures closely approximating their softening points during transfer or required transfer to an adhesive-coated substrate. This circumstance is explained in U.S. Pat. No. 3,688,695.
Another means for decorating glass, glass-ceramic, and ceramic ware which has been utilized extensively involves decalcomania transfers (decals). Three principal types have been used in commerce: the water release-type, the heat release-type, and the pressure release- or cold release-type.
As the name indicates, the first type requires soaking the decal with water to effect separation of the backing from the design. The design is temporarily adhered to the substrate, the substrate thoroughly dried, and then fired at a sufficiently high temperature to fuse the design into the surface thereof.
The heat release-type decal comprises a heat releasable backing at the face of the vitreous design layer and an outermost thermoplastic or heat activatable adhesive surface at the opposite or rear side of the vitreous design. The thermoplastic side of the decal is pressed against the surface of a preheated substrate whereby the heat from the substrate softens the adhesive surface sufficiently to cause the design to be temporarily attached to the substrate. In a second action occurring concurrently, the heat from the substrate softens the heat releasable backing, thereby effecting release of the backing from the design. The substrate is subsequently fired in the normal manner to fuse the design into the surface thereof.
The pressure release-type decalcomania can have a structure similar to that of the heat release-type except that the heat release layer is replaced with a silicone release layer. Moreover, the adhesive layer overlay must exhibit room temperature pressure sensitivity, thereby removing any need for heating the substrate. The adhesive layer manifests a preferential adherence to the substrate rather than the silicone release layer, which enables release of the decal to be achieved by simply pressing the decal against the ware to be decorated.
The term "thermoplastic," as discussed in the context of this adhesive layer, must be distinguished from the expression as utilized above in relation to thermoplastic inks. Under both sets of circumstances, the materials reversibly soften upon the application of heat, thus giving rise to the term "thermoplastic." When employed in the context of inks, however, the expression also implies melt processibility or application; whereas the material suitable for the decal adhesive layer is simply applied from solution over the vitreous design. Therefore, to remove possible ambiguity in the expression "thermoplastic," melt processible inks have been referred to as "hot melt inks" or merely "hot color;" whereas the solution-applied adhesive overlay has frequently been designated as a "lacquer."
U.S. application Ser. No. 74,907, pending filed concurrently herewith by the present applicant, discloses a number of thermoplastic ink formulations suitable for use in heat release-type and pressure release-type decals. That specification also provides a more detailed description of the construction of heat release-type and pressure release-type decalcomania and reference is hereby made to that description. The inks disclosed therein consist generally of:
(a) a vitrifiable, inorganic flux optionally containing inorganic pigments; PA1 (b) an ethylene-ester copolymer resin having a copolymerized ester selected from the group of vinyl alcohol esters of C.sub.1 -C.sub.4 saturated monocarboxylic acids and C.sub.1 -C.sub.5 saturated alcohol esters of acrylic or methacrylic acid; PA1 (c) an amorphous tackifying resin; PA1 (d) an organic wax and/or plasticizer; and, optionally, PA1 (e) a modifying polyolefin resin. PA1 (a) about 50-80% by weight of a vitrifiable, inorganic flux containing up to about 40% by weight inorganic pigments and opacifiers; PA1 (b) about 5-25% by weight of a polyoxyalkylene ether having a molecular weight greater than 200 but less than 10,000; PA1 (c) about 2-20% by weight of a water soluble, cohesive strength imparting polymer having a molecular weight between about 25,000-500,000; and, optionally, PA1 (d) up to 20% by weight of a modifying wax and/or plasticizer selected from the group consisting of a polyoxyalkylene ether wax having a softening (melt) point between about 35.degree.-65.degree. C. and a molecular weight between about 1000-25,000 and a compatible ester having a softening (melt) point below about 65.degree. C.